This invention relates generally to fiber optics and more particularly to a coupler for coupling single core optical fibers to a concentric core optical fiber.
Fiber optics is currently finding wide application in communications equipment because of its ability to convey modulated light energy and due to the bandwidth considerations involved, a great deal of information can be transmitted on a relatively small size optical fiber. While single core optical fibers have conventionally been used, concentric core fibers have been shown to be potentially attractive for use in intrusion resistant communication links. As is well known, in concentric core optical fibers, two separate and distinct coaxial inner and outer light conducting cores exist, being separated from one another by cladding material so that optical power can be transmitted via the cores with minimal crosstalk and attenuation. For many intrusion resistant configurations, it becomes necessary to couple separate signals from separate light sources into the two cores independently. To do this requires an input coupler capable of low crosstalk and low insertion losses.
One known coupler for concentric core fibers comprises the use of two small step index fibers with core diameters equal to or less than the respective core thicknesses which are precisely aligned to the two concentric fiber cores and bonded into place by means of an epoxy. Although the crosstalk in such configuration is found to be within acceptable tolerance, considerable improvement is needed to improve the utility of the device in an intrusion resistant system. Although the insertion loss of such a device has been found to be tolerable, the small size of the input fibers, however, dictates that only a few microwatts of optical power can be coupled into either of the concentric core fibers. Additionally, large portions of the concentric cores, particularly the outer core, are not even illuminated and the capability of launching tens of microwatts from light emitting sources, such as diodes, is absent.
Accordingly, it is an object of the present invention to provide an improvement in optical fiber couplers.
Still another object of the present invention is to provide an improvement in concentric core optical fiber couplers which overcomes the limitation of crosstalk and input power capability.
These and other objects of the present invention are accomplished by the fabrication method and resulting structure wherein a plurality of single core input fibers are bonded to the outer core of a coaxial core optical fiber at a transition region where the outer core is exposed. The bonding is accomplished by transparent epoxy having an index of a fraction matching the cores. Also, opaque light absorbing material is also applied adjacent the location of bonding for absorbing extraneous optical power not coupled between the outer core and the plurality of single core input fibers. Additionally, at least one single core input fiber is aligned with the inner core of the concentric core fiber and bonded in end to end relationship therewith. Another region of opaque material is provided adjacent the location of the latter bonding to attenuate any excess power that is not launched into the inner core but may be ultimately coupled into the outer core and thus be a source of crosstalk.